Heretofore one input section has been connected to one output setion by one transmission line for transmitting data from the former to the latter. Where a plurality of input and output sections are involved, therefore, it has been customary to associate or connect the respective input sections with the corresponding output sections by different transmission lines. A control section has been provided to input data at fixed times and output them at fixed times. In order to control a plurality of data input and output sections by the control section, it has been customary to connect the corresponding input and output sections by a plurality of transmission lines arranged in parallel with one another (FIG. 8). Where such an arrangement is made, a relatively high level of control is required for proper selection of I/O terminals or proper transmission of data. In general, microcomputers are used for the high level of control.
However, where microcomputers are used, it is also necessary to provide interfaces suitable for the microcomputers. Where such interfaces are used if the microcomputers are modified or replaced with others so as to increase the data transmission capability of the entire data link system, the interfaces must be accordingly modified or replaced. Also, in such a case, a new software must also be developed. Furthermore, if, for example, the data transmission system with the microcomputers is used for an air conditioning system in a building, the need to connect the microcomputers to each room may result in relatively longer transmission lines.